Stretched polytetrafluoroethylene moldings and process for production thereof

ABSTRACT

Disclosed are expanded polytetrafluoroethylene products, such as porous polytetrafluoroethylene tubes, having high axial tear strength and a process for their production. Each expanded polytetrafluoroethylene product has a microstructure, which comprises fibrils and nodes interconnected with each other by the fibrils, and has an axial tear strength of not lower than 6,000 gf/mm as calculated in accordance with the following formula: L/[T×(V/100)] where L (gf) is an axial tear load, T (mm) is a wall thickness, and V (%) is a volume ratio of resin. The process for the production of the expanded polytetrafluoroethylene product includes a high-speed extrusion step.

TECHNICAL FIELD

This invention relates to expanded polytetrafluoroethylene products, andmore specifically to expanded polytetrafluoroethylene products havinghigh axial tear strength despite their high porosity, and also to aprocess for their production. The expanded polytetrafluoroethyleneproducts according to the present invention are generally used asartificial blood vessels and sutures and for other applications in theform of tubes, filaments, rods or the like. Expandedpolytetrafluoroethylene tubes according to the present invention areequipped with suitable properties especially as artificial blood vesselmaterials.

BACKGROUND ART

An expanded polytetrafluoroethylene (hereinafter abbreviated as “PTFE”)product formed using PTFE has a microstructure comprising fibrils andnodes interconnected with each other by the fibrils. Expanded PTFEproducts are porous for such microstructures, and therefore, are alsocalled “porous PTFE products”

Properties of an expanded PTFE product, such as pore size and porosity,can be controlled primarily by relying upon stretching conditions. Amongsuch expanded PTFE products, expanded PTFE tubes which may hereinafterbe called “porous PTFE tubes” are provided with properties derived fromtheir porous nature, such as flexibility, fluid permeability, fineparticulate capturing capacity, low dielectric constant and lowdielectric dissipation factor, in addition to properties such as heatresistance and chemical resistance and surface properties such as lowfriction coefficient, water repellency and non-tackiness, all of whichare possessed by the material PTFE itself. For these unique properties,the utility of expanded PTFE tubes is not limited only to the generalindustrial field but is also spreading to the medical field and thelike.

Taking a porous PTFE tube, for example, it is rich in flexibility andits material PTFE itself is excellent in antithrombic property, andmoreover, its porous structure based on a microfibrous structure formedas a result of the stretching and comprising a number of fibrils and anumber of nodes interconnected with each other by the fibrils isexcellent in biocompatibility. Expanded PTFE tubes, therefore, havefound wide-spread utility as substitute blood vessels for maintainingcirculation, for example, to replace lesion parts of blood vessels inliving bodies, especially to bypass such lesion parts.

A porous PTFE tube is generally produced by mixing a liquid lubricantwith unsintered powder of PTFE, forming the resulting mixture into atubular shape by ram extrusion, drying off the liquid lubricant, andthen expanding the tubular extrusion product by stretching in thedirection of its axis. Subsequent to the expanding, the expandedextrusion product is heated to a temperature of the melting point ofPTFE or higher while holding it to avoid shrinkage, so that the expandedstructure is sintered and fixed. When the stretching temperature issufficiently high, the sintering and fixing is effected concurrentlywith the expanding step.

Despite such various excellent properties as mentioned above, porousPTFE tubes have a strong molecular orientation in the direction ofextrusion and tend to tear in the direction of their longitudinal axes.Porous PTFE tubes are, therefore, accompanied by a problem in that, whenblood vessels in living bodies are shunted using porous PTFE tubes asartificial blood vessels, the tubes may tear in the direction of theirlongitudinal axes by suture needles or sutures to induce hematomaformation or false aneurysm due to blood leakage. This problem becomesparticularly pronounced when upon production of porous PTFE tubes, thestretch ratio is increased to make the porosity higher, the pore size ismade greater, or the wall thickness is reduced.

As a method for providing an expanded PTFE tube with higher axial tearstrength, it may be contemplated to perform the stretching of anextrusion product in biaxial directions, that is, in the longitudinaldirection and in the radial direction. With this method alone, however,it is still impossible to achieve any substantial improvement in theaxial tear strength.

A process was therefore reported in JP-B-43-20384 and JP-B-7-15022.According to that process, extrusion is conducted while providing anextrusion product with an orientation at an angle with respect to thedirection of a longitudinal axis by producing a helical flow in theextrusion product with a helical groove formed on a die or mandrel of aram extruder.

In the above-described process, however, the extrusion product isstretched in the direction of its longitudinal axis in a subsequent stepso that the intersecting angle between the direction of the orientationand the direction of the longitudinal axis becomes too small to expectany substantial improvement in the axial tear strength. Especially whenstretching at a high stretch ratio of 4 times or more in thelongitudinal direction is needed to produce a porous PTFE tube having ahigh porosity of 70% or more, the direction of orientation becomescloser to the direction of the longitudinal axis so that practically noeffect is expected in increasing the axial tear strength.

With a view to obtaining a porous PTFE tube having high axial tearstrength, it was also proposed to reduce the porosity or to reinforce aporous PTFE tube by helically winding an expanded PTFE tape on an outersurface of the porous PTFE tube (JP-B-52-9074). Nowadays, one of twomethods is adopted, one being to lower the porosity of a porous PTFEtube, and the other being to helically wind an expanded PTFE tape orfilament on the outer surface of a porous PTFE tube such that the porousPTFE tube is reinforced.

However, the method, which relies upon a reduction in the porosity of aporous PTFE tube or a reinforcement by a tape or filament wound on theouter surface of a porous PTFE tube, involves a problem that someinherent characteristics of a porous PTFE tube, such as flexibility andtissue cells invasion, are impaired although the method is effective inincreasing the axial tear strength.

DISCLOSURE OF THE INVENTION

Objects of the present invention are to provide an expanded PTFEproduct, such as a porous PTFE tube, equipped with high axial tearstrength even without any reinforcement owing to an improvement in theaxial tear strength of the expanded PTFE product itself and also toprovide a process for its production. A further object of the presentinvention is to provide an artificial blood vessel made of a porous PTFEtube having high porosity and high axial tear strength.

The present inventors have proceeded with extensive research to achievethe above-described objects. As a result, it has been found that in astep of extruding a mixture of unsintered powder of PTFE and a lubricantinto a predetermined shape by a ram extruder, extrusion at a higherspeed than the conventional speed makes it possible to obtain anexpanded PTFE product pronouncedly improved in axial tear strength.

Based on the above finding, a high-speed ram extrusion technique and aram extruder suited for the technique have been also developed. Theirdevelopments have made it possible to stably produce an expanded PTFEproduct, such as a porous PTFE tube, having high porosity and high axialtear strength. This porous PTFE tube is equipped with excellentproperties especially as an artificial blood vessel. The presentinvention have been completed based on these findings.

According to the present invention, there is thus provided an expandedpolytetrafluoroethylene product having a microstructure comprisingfibrils and nodes interconnected with each other by the fibrils, whereinthe expanded polytetrafluoroethylene product has an axial tear strengthof not lower than 6,000 gf/mm as calculated in accordance with thefollowing formula: L/[T×(V/100)] where L (gf) is an axial tear load, T(mm) is a wall thickness, and V (%) is a volume ratio of resin.

The expanded polytetrafluoroethylene product can preferably be a porouspolytetrafluoroethylene tube. According to the present invention, anartificial blood vessel made of the porous polytetrafluoroethylene tubeis also provided.

According to the present invention, there is also provided a process forthe production of an expanded polytetrafluoroethylene product having anaxial tear strength of not lower than 6,000 gf/mm as calculated inaccordance with the following formula: L/[T×(V/100)] where L (gf) is anaxial tear load, T (mm) is a wall thickness, and V (%) is a volume ratioof resin (volume resin ratio), said process including the followingsteps: 1) extruding a mixture, which comprises unsintered powder ofpolytetrafluoroethylene and a lubricant, into a predetermined shape by aram extruder, 2) stretching the resulting extrusion product, and 3)sintering the thus-stretched product,

-   -   wherein in the step 1, the extrusion is conducted under such        conditions that an extrusion speed determined from a product of        an extrusion reduction ratio and a ram speed becomes not slower        than 19 m/min.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a conventional ram extruder.

FIG. 2 is a cross-sectional view of a ram extruder in which a cylinderand a master die are in the form of a jointless, continuous, unitarystructure unit.

BEST MODES FOR CARRYING OUT THE INVENTION

The expanded PTFE product according to the present invention can takevarious forms such as tubes, filaments and rods. Concerning an expandedPTFE tube (i.e., porous PTFE tube) as a representative example of suchforms, its production process will be described specifically.

To produce the porous PTFE tube, a lubricant is firstly mixed withunsintered powder of PTFE to prepare a mixture, the mixture is extrudedinto the form of a tube by using a ram extruder, and then, the tube isstretched at a desired stretch ratio in the direction of its axis. Theseprocedures can be conducted by following the process described inJP-B-42-13560, for example. While holding the resultant tube to avoidshrinkage, the tube is heated at its sintering temperature of 327° C. orhigher to sinter and fix the stretched structure. By this process, theporous PTFE tube can be obtained.

As the lubricant which is mixed as an aid, it is preferred to use aliquid lubricant which is in a liquid form at environmental temperature,such as naphtha. The liquid lubricant may be eliminated in thestretching step, although it is generally dried off subsequent to theextrusion step.

The porosity and fibril length of the porous PTFE can be set as desiredby adjusting its stretch ratio and stretch strain ratio. The stretchingis generally conducted by effecting unidirectional stretching. Thestretch ratio is selected generally from a range of from 1.2 to 15times, preferably from a range of from 2 to 10 times, more preferablyfrom a range of from 3 to 8 times. The sintering may be conducted whileeffecting stretching, although it may also be conducted subsequent tothe stretching step. To sinter the extrusion product while stretchingit, the extrusion product can be stretched, for example, in an electricfurnace controlled at 350 to 800° C. Conditions for the stretching andsintering after the extrusion step can be chosen as desired from theconditions known in this technical field.

To obtain the expanded PTFE product, such as the porous PTFE tube,having high axial tear strength according to the present invention, theextrusion speed determined as the product of the extrusion reductionratio, which may hereinafter be called “the extrusion RR”, and the ramspeed (mm/min) is set at not slower than 19 m/min, preferably not slowerthan 40 m/min in the extrusion step. It is preferred to set theextrusion RR at not smaller than 250 at this time, with not smaller than320 being more preferred. The upper limit of the extrusion RR can bepreferably 700, more preferably 650, especially preferably 600. Theupper limit of the extrusion speed is 100 m/min in general, although itis 70 m/min or so in many instances.

To improve the extrusion formability at high speed, it may be consideredpreferable to mix the liquid lubricant in a relatively high proportionrelative to the unsintered powder of PTFE. Mixing of the liquidlubricant in an excess amount, however, may lead to an expanded PTFEproduct of lowered strength. It is, therefore, desired to mix the liquidlubricant preferably in a proportion of not greater than 30 parts byweight, more preferably in a proportion of not greater than 26 parts byweight to 100 parts by weight of the unsintered powder of PTFE. Thelower limit of the proportion of the liquid lubricant may be preferably15 parts by weight, more preferably 18 parts by weight, especiallypreferably 20 parts by weight to 100 parts by weight of the unsinteredpowder of PTFE. It is desired to limit the amount of the liquidlubricant, which is to be added to 1 kg of the unsintered powder ofPTFE, preferably to 380 mL or less, more preferably to 330 mL or less.

When ram extrusion is conducted under the above-described conditions, adisturbance tends to occur in a resin flow so that a mandrel or core pinof a ram extruder may be off-centered, thereby possibly developing aproblem in quality to such an extent as the resulting extrusion productcannot be provided for actual use due to a reduction in the dimensionalaccuracy of the extrusion or the formation of damages in the form ofaxial grooves in an outer surface of the extrusion product. To overcomethis problem, it is important to make the content of the liquidlubricant in a paste (billet) before the ram extrusion and its densitydistribution as even as possible. With only these measures, however, nosufficient effects can be recognized for the lessening of the problem.

The present inventors pursued its cause. As a result, it was found thata small step and clearance inherently exist between a flange of a barreland that of a master die due to the structure of a ram extruder and theyact as primary causes of the disturbance in the resin flow. As aneffective method for the elimination of the problem, the presentinventors have developed a process for producing an extrusion product byusing a ram extruder in which a barrel and a master cylinder arecombined as a unitary structure.

As illustrated in FIG. 1, a conventional ram extruder is constructed ofan extrusion ram 11, a ram tip 12, a mandrel 13, a barrel 14, afastening clamp 15, a master die 16, a die 17, a die-fastening bolt 18,a core pin 19, and so on. In this ram extruder, the cylinder 14, themaster die 16 and the die 17 are discrete members, and their flanges arefastened by hydraulic clamps or bolts.

It has been found that, when high-speed extrusion is conducted using aconventional ram extruder such as that illustrated in FIG. 1, the resinpressure increases, a step and clearance hence become greater especiallybetween the flange of the cylinder 14 and that of the master die 16, andas a result, the disturbance in the resin flow becomes severer astypified by the formation of stagnated parts and peeled parts in theresin flow and the development of a leakage of the resin flow. Inaddition, it has also been found that the groove-shaped damages formedin the surface of the extrusion product occurred along the step formedat the flanges. To suppress the disturbance in the resin flow under thehigh-speed ram extrusion conditions, it is accordingly necessary tostructurally eliminate the step and clearance at the flanges of thebarrel and master die. Based on this finding, the present inventors havecontemplated the process for conducting high-speed extrusion by using aram extruder having a barrel and a master cylinder combined togetherinto a continuous unitary structure and suited for high-speed extrusion.

In the present invention, a ram extruder of such a structure as shown inFIG. 1 can be used. To obtain an expanded PTFE product having a smallcircumferential variation in thickness and more pronouncedly improvedaxial tear strength, however, it is preferred to use a ram extruderwhich, as illustrated in FIG. 2, is equipped with an extrusion jig 26formed of a barrel and a master die combined together into a continuousunitary structure with jointless smooth continuity therebetween andhaving a resin-contacting wall machined into a smooth surface bygrinding.

As shown in FIG. 2, the ram extruder in which the barrel and master dieare combined together into the jointless, continuous unitary structureis constructed of an extrusion ram 21, a ram tip 22, a mandrel 23, afastening clamp 24, a barrel housing 25, the extrusion jig (continuousunitary structure unit) 26 with the barrel and master die combinedtogether into the jointless unitary structure unit, a die 27, adie-fastening bolt 28, a core pin 29, and so on.

The continuous unitary structure unit 26 of the barrel and master die isdesirably formed with a material having high strength and hardnesssufficient to withstand the resin pressure and corrosion resistance, butit can be made of carbon steel or the like with hard andcorrosion-resistant chrome plating applied thereon. The resin (billet)with the liquid lubricant mixed therewith is loaded through an openingof the barrel portion of the continuous unitary structure unit 26, andthen, the continuous unitary structure unit 26 is mounted on a main bodyof the ram extruder. The main body of the ram extruder may preferably beequipped with the barrel housing 25 and a barrel lifter mechanism tofacilitate the mounting/dismounting of the continuous unitary structureunit 26.

The inner diameter of the barrel may set preferably such that theextrusion RR becomes not smaller than 250, with not smaller than 320being more preferred. The length of a linear section of the barrel canbe determined by a calculation from a resin volume which is required toobtain an extrusion product of a desired length. An appropriate value ofan internal angle of a tapered master die portion of the continuousunitary structure unit 26 and the die 27 can be set preferably at asgreat an internal angle as possible in view of a producible maximumresin pressure of the ram extruder from the standpoint of increasing theaxial tear strength further, although it varies depending on theextrusion conditions.

The expanded PTFE product according to the present invention canpreferably be in the form of a tube. This tube is a porous PTFE tube.When the porous PTFE tube according to the present invention is used asan artificial blood vessel material, the fibril length (average value)and porosity can be set preferably at not shorter than 20 μm and notlower than 60%, more preferably at not shorter than 40 μm and not lowerthan 70%, respectively, in order to enhance the tissue cells invasionsuch that the artificial blood vessel can be provided with equivalentfunctions to those of the blood vessels in the living body at an earlystage subsequent to its grafting and can remain open. The upper limit ofthe fibril length may be preferably 100 μm, more preferably 80 μm,especially preferably 60 μm. The upper limit of the porosity may bepreferably 90%, with 85% being more preferred. The wall thickness of thetube may be preferably from 200 to 1,000 μm, more preferably from 300 to900 μm, although it varies depending on the inner diameter.

The circumferential variation in the thickness of the expanded PTFEproduct may be preferably not greater than 45%, more preferably notgreater than 40%, still more preferably not greater than 35%. In fieldswhere expanded PTFE products of particularly small circumferentialvariations in the thickness are required, it is desired to control thecircumferential variation in the thickness to smaller than 15%, notablyto not greater than 10%.

When extrusion is performed under such conditions as achieving anextrusion speed of not slower than 19 m/min by using the ram extruder ofthe “discrete structure” illustrated in FIG. 1 that the barrel and themaster die are not combined into a unitary structure, thecircumferential variation in the thickness of the finally-availableexpanded PTFE product generally falls within a range of from 15 to 45%or so, and therefore, becomes relatively large.

When extrusion is performed under such conditions as achieving anextrusion speed of not slower than 19 m/min by using a ram extruderequipped with a barrel and a master die combined together into acontinuous unitary structure with jointless smooth continuitytherebetween as illustrated in FIG. 2, on the other hand, thecircumferential variation in the thickness of the finally-availableexpanded PTFE product can be controlled to fall preferably within arange of 3% or greater but smaller than 15%, more preferably within arange of from 4 to 13%, especially preferably within a range of from 5to 10%.

The axial tear strength of the stretch PTFE product can be calculated inaccordance with the following formula: L/[T×(V/100)] where L (gf) is anaxial tear load, T (mm) is a wall thickness, and V (%) is a volume ratioof resin. The axial tear strength is not lower than 6,000 gf/mm,preferably not lower than 6,500 gf/mm, more preferably not lower than7,000 gf/mm. The upper limit of the axial tear strength is generally12,000 gf/mm, but in many instances, is 10,000 gf/mm or so.

In addition to a tube, the expanded PTFE product according to thepresent invention can also take the form of a filament, rod or the like.Further, the expanded PTFE product can also take the form of anelongated, expanded PTFE tape. As the expanded PTFE product according tothe present invention is equipped with axial tear strength pronouncedlyimproved compared with conventional products, it is preferred to obtainthe expanded PTFE product in the form of a tube and to use it as anartificial blood vessel material of a desired diameter.

EXAMPLES

The present invention will hereinafter be described more specificallybased on examples and comparative examples. The following methods wereemployed for the measurement, ranking and calculation of physicalproperties and other characteristic properties.

(1) Extrusion Reduction Ratio (Extrusion RR)

An extrusion reduction ratio was calculated in accordance with thefollowing formula:Extrusion RR=(D ₁ ² −D ₂ ²)÷(d ₁ ² −d ₂ ²)

-   -   where    -   D₁: extruder barrel diameter,    -   D₂: extruder mandrel diameter,    -   d₁: extruder die diameter, and    -   d₂: extruder core pin diameter.        (2) Extrusion Speed

An extrusion speed was calculated in accordance with the followingformula:Extrusion speed (mg/min)=(extrusion RR)×[ram speed (m/min)](3) Wall Thickness and Circumferential Variation in Thickness

An expanded PTFE tube was embedded in a paraffin block which had beencolored black with a dye. Subsequently, the paraffin block was shavedwith a microtome to have one cross-section of the expanded PTFE tubeexposed in an outer surface of the paraffin block so that a sample wasprepared for the observation of a wall thickness. Under astereomicroscope, the wall thickness of that sample was measured at fourpoints (n=4) along the circumference thereof, and an average of thevalues so measured was recorded as its wall thickness. From those fourmeasurement values, a circumferential variation in thickness wasdetermined in accordance with the following formula:Circumferential variation in thickness (%)=[(largest thickness−smallestthickness)÷wall thickness]×100(4) Porosity

A porosity was determined in accordance with ASTM D-792. Uponcalculation of the porosity, 2.25 g/cc was used as the true specificgravity of PTFE.

(5) Volume Ratio of Resin

A volume ratio of resin was calculated in accordance with the followingformula:Volume ratio of resin=100−porosity (%)(6) Fibril Length

Under a scanning electron microscope (SEM), an inner wall of an expandedPTFE tube was observed. Selected were thirty or more fibrils, whichexisted in an area of a rectangular visual field of 300 μm or greater inwidth along the radial direction and 400 μm or greater in length alongthe axial direction, and their lengths were measured. Extracting the top10% of all the fibrils from the one having the longest measured value,their average value was recorded as a fibril length.

(7) Axial Tear Load (gf)

An expanded PTFE tube was cut crosswise along a plane which isperpendicular to the axis of the tube. At a position 3 mm apart from anedge of a cut end of the tube in the direction of the axis of the tube,a round needle of 0.4 mm in outer diameter was pierced at right anglesthrough the wall of the tube to form through-holes. The tube was nextfixed at an opposite end thereof, where no through-holes had beenformed, on a grip attached to a load cell of a uniaxial tensile testingmachine (“Autograph AG500E”, manufactured by Shimadzu Corporation). Amild steel wire of 0.2 mm in outer diameter was caused to extend throughthe through-holes, and was then fixed at opposite ends thereof onanother grip fixedly secured on a crosshead. At a crosshead speed of 20mm/min, the wire was pulled in the axial direction of the tube. Here,the crosshead was caused to undergo continuous displacements until thetube was completely torn up and the wire separated from the tube. Amaximum load at this time was recorded as an axial tear load (gf).

(8) Axial Tear Strength (gf/mm)

An axial tear strength can be calculated in accordance with thefollowing formula: L/[T×(V/100)] where L (gf) is an axial tear load, T(mm) is a wall thickness, and V (%) is a volume ratio of resin. Morespecifically, the axial tear strength was calculated in accordance withbelow-described formula. Calculated was an average value of five samplesmeasured (n=5).Axial tear strength (gf/mm)=[axial tear load (gf)]÷[wall thickness(mm)]÷[volume ratio of resin÷100]

Example 1

Naphtha (“DRYSOL”, product of Esso SS; 26 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were discrete from eachother, the mixture was then extruded into the form of a tube under thefollowing conditions: 90 mm barrel diameter, 21 mm mandrel diameter, 4.8mm die diameter, 3 mm core pin diameter, and 60 mm/min ram speed.Subsequently, naphtha was dried off from the tube over 48 hours in aconstant-temperature chamber controlled at 60° C. The tube was sinteredin an electric furnace controlled at 600° C. while stretching it at astretch ratio of 4.5 times to obtain a porous PTFE tube having 75%porosity and 53 μm fibril length.

Example 2

Naphtha (“DRYSOL”, product of Esso SS; 23 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were discrete from eachother, the mixture was then extruded into the form of a tube under thefollowing conditions: 90 mm barrel diameter, 21 mm mandrel diameter, 6.3mm die diameter, 4.5 mm core pin diameter, and 60 mm/min ram speed.Subsequently, naphtha was dried off from the tube over 48 hours in aconstant-temperature chamber controlled at 60° C. The tube was sinteredin an electric furnace controlled at 600° C. while stretching it at astretch ratio of 4.5 times to obtain a porous PTFE tube having 74%porosity and 45 μm fibril length.

Example 3

Naphtha (“DRYSOL”, product of Esso SS; 22 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were discrete from eachother, the mixture was then extruded into the form of a tube under thefollowing conditions: 90 mm barrel diameter, 21 mm mandrel diameter,7.26 mm die diameter, 5.35 mm core pin diameter, and 60 mm/min ramspeed. Subsequently, naphtha was dried off from the tube over 48 hoursin a constant-temperature chamber controlled at 60° C. The tube wassintered in an electric furnace controlled at 600° C. while stretchingit at a stretch ratio of 4.5 times to obtain a porous PTFE tube having74% porosity and 43 μm fibril length.

Example 4

Naphtha (“DRYSOL”, product of Esso SS; 23 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were discrete from eachother, the mixture was then extruded into the form of a tube under thefollowing conditions: 90 mm barrel diameter, 21 mm mandrel diameter, 6.3mm die diameter, 4.5 mm core pin diameter, and 150 mm/min ram speed.Subsequently, naphtha was dried off from the tube over 48 hours in aconstant-temperature chamber controlled at 60° C. The tube was sinteredin an electric furnace controlled at 600° C. while stretching it at astretch ratio of 4.5 times to obtain a porous PTFE tube having 75%porosity and 52 μm fibril length.

Example 5

Naphtha (“DRYSOL”, product of Esso SS; 22 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were discrete from eachother, the mixture was then extruded into the form of a tube under thefollowing conditions: 90 mm barrel diameter, 21 mm mandrel diameter,7.26 mm die diameter, 5.35 mm core pin diameter, and 150 mm/min ramspeed. Subsequently, naphtha was dried off from the tube over 48 hoursin a constant-temperature chamber controlled at 60° C. The tube wassintered in an electric furnace controlled at 600° C. while stretchingit at a stretch ratio of 4.5 times to obtain a porous PTFE tube having72% porosity and 48 μm fibril length.

Example 6

Naphtha (“DRYSOL”, product of Esso SS; 25.5 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were discrete from eachother, the mixture was then extruded into the form of a tube under thefollowing conditions: 130 mm barrel diameter, 50 mm mandrel diameter,8.6 mm die diameter, 6.4 mm core pin diameter, and 60 mm/min ram speed.Subsequently, naphtha was dried off from the tube over 48 hours in aconstant-temperature chamber controlled at 60° C. The tube was sinteredin an electric furnace controlled at 600° C. while stretching it at astretch ratio of 6 times to obtain a porous PTFE tube having 79%porosity and 40 μm fibril length.

Example 7

Naphtha (“DRYSOL”, product of Esso SS; 25.5 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were combined together in acontinuous unitary form, the mixture was then extruded into the form ofa tube under the following conditions: 130 mm barrel diameter, 50 mmmandrel diameter, 8.6 mm die diameter, 6.4 mm core pin diameter, and 60mm/min ram speed. Subsequently, naphtha was dried off from the tube over48 hours in a constant-temperature chamber controlled at 60° C. The tubewas sintered in an electric furnace controlled at 600° C. whilestretching it at a stretch ratio of 6 times to obtain a porous PTFE tubehaving 79% porosity and 43 μm fibril length.

Example 8

Naphtha (“DRYSOL”, product of Esso SS; 25.5 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were combined together in acontinuous unitary form, the mixture was then extruded into the form ofa tube under the following conditions: 130 mm barrel diameter, 50 mmmandrel diameter, 8.6 mm die diameter, 6.4 mm core pin diameter, and 150mm/min ram speed. Subsequently, naphtha was dried off from the tube over48 hours in a constant-temperature chamber controlled at 60° C. The tubewas sintered in an electric furnace controlled at 600° C. whilestretching it at a stretch ratio of 6 times to obtain a porous PTFE tubehaving 81% porosity and 58 μm fibril length.

Example 9

Naphtha (“DRYSOL”, product of Esso SS; 25.5 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were combined together in acontinuous unitary form, the mixture was then extruded into the form ofa tube under the following conditions: 130 mm barrel diameter, 21 mmmandrel diameter, 11.0 mm die diameter, 8.6 mm core pin diameter, and 60mm/min ram speed. Subsequently, naphtha was dried off from the tube over48 hours in a constant-temperature chamber controlled at 60° C. The tubewas sintered in an electric furnace controlled at 600° C. whilestretching it at a stretch ratio of 6 times to obtain a porous PTFE tubehaving 80% porosity and 48 μm fibril length.

Example 10

Naphtha (“DRYSOL”, product of Esso SS; 25.5 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were combined together in acontinuous unitary form, the mixture was then extruded into the form ofa tube under the following conditions: 130 mm barrel diameter, 21 mmmandrel diameter, 11.0 mm die diameter, 8.6 mm core pin diameter, and150 mm/min ram speed. Subsequently, naphtha was dried off from the tubeover 48 hours in a constant-temperature chamber controlled at 60° C. Thetube was sintered in an electric furnace controlled at 600° C. whilestretching it at a stretch ratio of 6 times to obtain a porous PTFE tubehaving 81% porosity and 55 μm fibril length.

Comparative Example 1

Naphtha (“DRYSOL”, product of Esso SS; 26 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were discrete from eachother, the mixture was then extruded into the form of a tube under thefollowing conditions: 90 mm barrel diameter, 21 mm mandrel diameter, 4.8mm die diameter, 3 mm core pin diameter, and 30 mm/min ram speed.Subsequently, naphtha was dried off from the tube over 48 hours in aconstant-temperature chamber controlled at 60° C. The tube was sinteredin an electric furnace controlled at 600° C. while stretching it at astretch ratio of 4.5 times to obtain a porous PTFE tube having 73%porosity and 51 μm fibril length.

Comparative Example 2

Naphtha (“DRYSOL”, product of Esso SS; 23 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were discrete from eachother, the mixture was then extruded into the form of a tube under thefollowing conditions: 90 mm barrel diameter, 21 mm mandrel diameter, 6.3mm die diameter, 4.5 mm core pin diameter, and 30 mm/min ram speed.Subsequently, naphtha was dried off from the tube over 48 hours in aconstant-temperature chamber controlled at 60° C. The tube was sinteredin an electric furnace controlled at 600° C. while stretching it at astretch ratio of 4.5 times to obtain a porous PTFE tube having 73%porosity and 48 μm fibril length.

Comparative Example 3

Naphtha (“DRYSOL”, product of Esso SS; 22 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were discrete from eachother, the mixture was then extruded into the form of a tube under thefollowing conditions: 90 mm barrel diameter, 21 mm mandrel diameter,7.26 mm die diameter, 5.35 mm core pin diameter, and 30 mm/min ramspeed. Subsequently, naphtha was dried off from the tube over 48 hoursin a constant-temperature chamber controlled at 60° C. The tube wassintered in an electric furnace controlled at 600° C. while stretchingit at a stretch ratio of 4.5 times to obtain a porous PTFE tube having74% porosity and 43 μm fibril length.

Comparative Example 4

Naphtha (“DRYSOL”, product of Esso SS; 21.5 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were discrete from eachother, the mixture was then extruded into the form of a tube under thefollowing conditions: 90 mm barrel diameter, 21 mm mandrel diameter, 8.6mm die diameter, 6.4 mm core pin diameter, and 60 mm/min ram speed.Subsequently, naphtha was dried off from the tube over 48 hours in aconstant-temperature chamber controlled at 60° C. The tube was sinteredin an electric furnace controlled at 600° C. while stretching it at astretch ratio of 6 times to obtain a porous PTFE tube having 80%porosity and 35 μm fibril length.

Comparative Example 5

Naphtha (“DRYSOL”, product of Esso SS; 18.5 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were discrete from eachother, the mixture was then extruded into the form of a tube under thefollowing conditions: 90 mm barrel diameter, 21 mm mandrel diameter,11.0 mm die diameter, 8.6 mm core pin diameter, and 60 mm/min ram speed.Subsequently, naphtha was dried off from the tube over 48 hours in aconstant-temperature chamber controlled at 60° C. The tube was sinteredin an electric furnace controlled at 600° C. while stretching it at astretch ratio of 6 times to obtain a porous PTFE tube having 79%porosity and 20 μm fibril length.

Comparative Example 6

Naphtha (“DRYSOL”, product of Esso SS; 18.5 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were discrete from eachother, the mixture was then extruded into the form of a tube under thefollowing conditions: 90 mm barrel diameter, 21 mm mandrel diameter,11.0 mm die diameter, 8.6 mm core pin diameter, and 6 mm/min ram speed.Subsequently, naphtha was dried off from the tube over 48 hours in aconstant-temperature chamber controlled at 60° C. The tube was sinteredin an electric furnace controlled at 600° C. while stretching it at astretch ratio of 6 times to obtain a porous PTFE tube having 77%porosity and 18 μm fibril length.

Comparative Example 7

Naphtha (“DRYSOL”, product of Esso SS; 18.5 parts by weight) was mixedwith PTFE fine powder (“POLYFLON F104”, product of Daikin Industries,Ltd.), and the resulting mixture was left over for 2 hours in aconstant-temperature chamber controlled at 30° C. Using a ram extruderof the type that a barrel and a master die were discrete from eachother, the mixture was then extruded into the form of a tube under thefollowing conditions: 90 mm barrel diameter, 21 mm mandrel diameter,11.0 mm die diameter, 8.6 mm core pin diameter, and 100 mm/min ramspeed. Subsequently, naphtha was dried off from the tube over 48 hoursin a constant-temperature chamber controlled at 60° C. The tube wassintered in an electric furnace controlled at 600° C. while stretchingit at a stretch ratio of 6 times to obtain a porous PTFE tube having 80%porosity and 22 μm fibril length.

Comparative Examples 8-17

Commercially-available, ten (10) artificial blood vessels were measuredfor properties consisting of porosity, fibril length, wall thickness andaxial tear strength. The artificial blood vessels of W.L. Gore &Associates' Medical Product Division, Baxter Healthcare Corporation andMeadox Medicals Inc. were reinforced by tape-shaped or filament-shapedPTFE wound on outer surfaces of expanded PTFE tubes, respectively. Itwas possible to remove those reinforcements without damaging theexpanded PTFE tubes which were main bodies of the artificial bloodvessels. Concerning the artificial blood vessels of W.L Gore &Associates' Medical Product Division, Baxter Healthcare Corporation andMeadox Medicals Inc., the properties of their expanded PTFE tubeswithout the reinforcements were ranked. TABLE 1 Example 1 Example 2Example 3 Example 4 Example 5 Production conditions Proportion of 26 2322 23 22 extrusion aid (parts by weight) Structure of ram DiscreteDiscrete Discrete Discrete Discrete extruder barrel and barrel andbarrel and barrel and barrel and master die master die master die masterdie master die Barrel diameter (mm) 90 90 90 90 90 Mandrel diameter (mm)21 21 21 21 21 Die diameter (mm) 4.8 6.3 7.26 6.3 7.26 Core pin diameter(mm) 3 4.5 5.35 4.5 5.35 Extrusion RR 546 394 318 394 318 Ram speed(m/min) 60 60 60 150 150 Extrusion speed 33 24 19 59 48 (m/min) Stretchratio 4.5 4.5 4.5 4.5 4.5 Ranking results of Circumferential 24 18 22 2826 properties variation in thickness Fibril length (μm) 53 45 43 52 48Porosity (%) 75 74 74 75 72 Volume ratio of resin 25 26 26 25 28 Averagewall thickness 672 683 715 675 737 (μm) Axial tear load (gf) 1148 10781123 1216 1355 Axial tear strength 6833 6071 6041 7206 6566 (gf/mm)Example 6 Example 7 Example 8 Example 9 Example 10 Production conditionsProportion of extrusion 25.5 25.5 25.5 25.5 25.5 aid (parts by weight)Structure of ram Discrete Continuous Continuous Continuous Continuousextruder barrel and unitary unitary unitary unitary master die barreland barrel and barrel and barrel and master die master die master diemaster die Barrel diameter (mm) 130 130 130 130 130 Mandrel diameter(mm) 50 50 50 21 21 Die diameter (mm) 8.6 8.6 8.6 11 11 Core pindiameter (mm) 6.4 6.4 6.4 8.6 8.6 Extrusion RR 436 436 436 350 350 Ramspeed (m/min) 60 60 150 60 150 Extrusion speed (m/min) 26 26 65 21 53Stretch ratio 6 6 6 6 6 Ranking results of Circumferential 35 6 6 7 8properties variation in thickness Fibril length (μm) 40 43 58 48 55Porosity (%) 79 79 81 80 81 Volume ratio of resin 21 21 19 20 19 Averagewall thickness 785 775 778 846 851 (μm) Axial tear load (gf) 1087 12501444 1158 1432 Axial tear strength 6594 7680 9769 6844 8856 (gf/mm)

TABLE 2 Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3 Comp. Ex. 4 Productionconditions Proportion of extrusion aid 26 23 22 21.5 (parts by weight)Structure of ram extruder Discrete Discrete Discrete Discrete barrel andbarrel and barrel barrel master die master die and and master master diedie Barrel diameter (mm) 90 90 90 90 Mandrel diameter (mm) 21 21 21 21Die diameter (mm) 4.8 6.3 7.26 8.6 Core pin diameter (mm) 3 4.5 5.35 6.4Extrusion RR 546 394 318 232 Ram speed (m/min) 30 30 30 60 Extrusionspeed (m/min) 16 12 10 14 Stretch ratio 4.5 4.5 4.5 6 RankingCircumferential variation in 15 7 4 6 results of thickness propertiesFibril length (μm) 51 48 43 35 Porosity (%) 73 73 74 80 Volume ratio ofresin 27 27 26 20 Average wall thickness (μm) 672 678 730 783 Axial tearload (gf) 953 697 635 350 Axial tear strength (gf/mm) 5252 3807 33462235 Comp. Ex. 5 Comp. Ex. 6 Comp. Ex. 7 Production conditionsProportion of extrusion aid 18.5 18.5 18.5 (parts by weight) Structureof ram extruder Discrete Discrete Discrete barrel and barrel and barreland master die master die master die Barrel diameter (mm) 90 90 90Mandrel diameter (mm) 21 21 21 Die diameter (mm) 11 11 11 Core pindiameter (mm) 8.6 8.6 8.6 Extrusion RR 163 163 163 Ram speed (m/min) 606 100 Extrusion speed (m/min) 10 1 16 Stretch ratio 6 6 6 RankingCircumferential variation in 7 8 10 results of thickness propertiesFibril length (μm) 20 18 22 Porosity (%) 79 77 80 Volume ratio of resin21 23 20 Average wall thickness (μm) 854 848 852 Axial tear load (gf)450 375 678 Axial tear strength (gf/mm) 2509 1923 3979

TABLE 3 Comp. Ex. 8 Comp. Ex. 9 Comp. Ex. 10 Comp. Ex. 11 Comp. Ex. 12Maker Gore Gore Impra Impra Atrium Manner of reinforcement ReinforcementReinforcement Low Low Low for artificial blood by by porosity porosityporosity vessel tape tape winding winding Axial tear load of 921 891 873764 533 artificial blood vessel (gf) Properties of expanded Fibrillength (μm) Not 24 Not 21 Not PTFE tube measured measured measuredPorosity (%) 74 73 66 70 59 Volume ratio of 26 27 34 30 41 resin Averagewall 413 639 445 598 515 thickness (μm) Axial tear load (gf) 342 526 783764 533 Axial tear strength 3185 3049 5175 4259 2524 (gf/mm) Comp. Ex.13 Comp. Ex. 14 Comp. Ex. 15 Comp. Ex. 16 Comp. Ex. 17 Maker AtriumMeadox Meadox Baxter Baxter Manner of reinforcement for Low Low LowReinforcement Reinforcement artificial blood vessel porosity porosity,porosity, by by reinforcement reinforcement tape tape by by windingwinding filament filament winding winding Axial tear load of 664 803 Not1086 Not artificial blood vessel (gf) measured measured Properties ofexpanded Fibril length (μm) 30 Not 29 Not 22 PTFE tube measured measuredPorosity (%) 62 63 64 70 72 Volume ratio of resin 38 37 36 30 28 Averagewall thickness 638 442 648 419 632 (μm) Axial tear load (gf) 664 5981231 414 684 Axial tear strength 2738 3657 5276 3294 3865 (gf/mm)

In each of Examples 1-10 where the extrusion speed was not slower than19 m/min as a production condition, the axial tear strength was higherthan 6,000 gf/mm. It is, therefore, understood that the porous PTFEtubes of these examples were substantially improved in axial tearstrength over those of the comparative examples. When the conventionalram extruder of the discrete barrel/master die type was used (Examples1-6), the setting of the extrusion speed at not slower than 19 m/minresulted in a high circumferential variation in thickness of not lowerthan 18%. When the ram extruder of the continuous unitary cylinder/dietype was used (Examples 7-10), however, the circumferential variation inthe thickness was significantly reduced. The setting of the extrusionspeed at not slower than 19 m/min as a production condition can,therefore, produce a porous PTFE tube having high porosity, long fibrillength and high axial tear strength and suited as an artificial bloodvessel.

INDUSTRIAL APPLICABILITY

According to the present invention, expanded PTFE products, such asporous PTFE tubes, pronouncedly improved in their own axial tearstrength are provided. Porous PTFE tubes according to the presentinvention have high axial tear strength even without any reinforcement,and show excellent properties especially as artificial blood vesselmaterials. According to the present invention, porous PTFE tubes havinghigh porosity and high axial tear strength can be provided.

1. An expanded polytetrafluoroethylene product having a microstructurecomprising fibrils and nodes interconnected with each other by saidfibrils, wherein said expanded polytetrafluoroethylene product has anaxial tear strength of not lower than 6,000 gf/mm as calculated inaccordance with the following formula: L/[T×(V/100)] where L (gf) is anaxial tear load, T (mm) is a wall thickness, and V (%) is a volume ratioof resin.
 2. An expanded polytetrafluoroethylene product according toclaim 1, wherein said axial tear strength is not lower than 7,000 gf/mm.3. An expanded polytetrafluoroethylene product according to claim 1,wherein said expanded polytetrafluoroethylene product has a fibrillength of not shorter than 20 μm and a porosity of not lower than 60%.4. An expanded polytetrafluoroethylene product according to claim 1,wherein said expanded polytetrafluoroethylene product has a fibrillength of not shorter than 40 μm and a porosity of not lower than 70%.5. An expanded polytetrafluoroethylene product according to claim 1,wherein said expanded polytetrafluoroethylene product has acircumferential variation in thickness of not greater than 45%.
 6. Anexpanded polytetrafluoroethylene product according to claim 1, whereinsaid expanded polytetrafluoroethylene product has a circumferentialvariation in thickness of smaller than 15%.
 7. An expandedpolytetrafluoroethylene product according to claim 1, wherein saidexpanded polytetrafluoroethylene product has a wall thickness in a rangeof from 200 to 1,000 μm.
 8. An expanded polytetrafluoroethylene productaccording to claim 1, wherein said expanded polytetrafluoroethyleneproduct has been obtained by a process which comprises extruding amixture, which comprises unsintered powder of polytetrafluoroethyleneand a lubricant, into a predetermined shape by a ram extruder under suchconditions that an extrusion speed determined from a product of anextrusion reduction ratio and a ram speed becomes not lower than 19m/min, stretching the resulting extrusion product, and then sinteringthe thus-stretched product.
 9. An expanded polytetrafluoroethyleneproduct according to claim 1, which is in a form of a tube.
 10. Anexpanded polytetrafluoroethylene product according to claim 9, whereinsaid tube is an artificial blood vessel material.
 11. A process for theproduction of an expanded polytetrafluoroethylene product having anaxial tear strength of not lower than 6,000 gf/mm as calculated inaccordance with the following formula: L/[T×(V/100)] where L (gf) is anaxial tear load, T (mm) is a wall thickness, and V (%) is a volume ratioof resin, said process including the following steps: 1) extruding amixture, which comprises unsintered powder of polytetrafluoroethyleneand a lubricant, into a predetermined shape by a ram extruder, 2)stretching the resulting extrusion product, and 3) sintering thethus-stretched product, wherein in said step 1, said extrusion isconducted under such conditions that an extrusion speed determined froma product of an extrusion reduction ratio and a ram speed becomes notslower than 19 m/min.
 12. A process according to claim 11, wherein insaid step 1, said extrusion is conducted using a ram extruder in which abarrel and a master die is in a form of a Pointless, continuous, unitarystructure unit.
 13. A process according to claim 11, wherein in saidstep 1, an extrusion reduction ratio is set at not smaller than
 250. 14.A process according to claim 11, wherein in said step 1, a mixturecomprising not more than 30 parts by weight of said lubricant per 100parts by weight of said unsintered powder of polytetrafluoroethylene isused.
 15. A process according to claim 11, wherein an expandedpolytetrafluoroethylene product having a fibril length of not shorterthan 20 μm and a porosity of not lower than 60% is obtained.
 16. Aprocess according to claim 11, wherein an expandedpolytetrafluoroethylene product having a fibril length of not shorterthan 40 μm and a porosity of not lower than 70% is obtained.
 17. Aprocess according to claim 11, wherein an expandedpolytetrafluoroethylene product having a circumferential variation inthickness of not greater than 45% is obtained.
 18. A process accordingto claim 11, wherein an expanded polytetrafluoroethylene product havinga circumferential variation in thickness of smaller than 15% isobtained.
 19. A process according to claim 11, wherein an expandedpolytetrafluoroethylene product having a wall thickness in a range offrom 200 to 1,000 μm is obtained.
 20. A process according to claim 11,wherein an expanded polytetrafluoroethylene product in a form of a tubeis obtained.